This invention relates to a semiconductor device and a method of manufacturing the same, and in particular, to a method of forming an interlayer insulating film of a MOSFET.
When a bias voltage is applied to a MOSFET, a threshold voltage, a mutual induction and an on-current are often variable with time. This phenomenon is generally called a hot carrier effect that reduces the reliability of a device.
Recently, the hot carrier effect often becomes remarkable when a gate length of the device is less than 1 .mu.m. This hot carrier effect becomes a serious restriction factor to miniaturize the MOSFET. In particular, a gate oxide film is often destroyed by the hot carrier effect. The destruction of the gate oxide film becomes a large problem to miniaturize the transistor, and will be thereinafter referred to as a hot carrier deterioration.
The hot carrier effect is mainly caused by Si--H combinations which become the cause of the hot carrier deterioration. More specifically, water (H.sub.2 O) diffuses into the gate oxide film to increase the Si--H combinations. This fact has been described in the 48th symposium lecture with respect to a semiconductor integrated circuit technique (water diffusion model for an increase effect of the hot carrier deterioration due to a nitride film passivation, P.134).
To avoid the above problem, a method has been suggested for preventing invasion of the water into a LSI by using the silicon nitride (SiN) film. In this event, the silicon nitride film has an excessively small diffusion factor against the water as a protection film of a parasitic-mold LSI.
A semiconductor device generally has silicon oxide films for device separation and diffusion layers or regions in a silicon substrate, and will be referred to as a first conventional reference. Further, the gate oxide film is deposited between the diffusion layers and on the silicon substrate. Further, a gate electrode is placed on the gate oxide film. Moreover, spacer oxide films are placed at both side surfaces of the gate electrode.
In this event, metal silicide layers each of which has a high melting point are formed on the surfaces of the diffusion layers and on the gate electrode, respectively. The metal silicide layer often becomes essential to miniaturize the semiconductor device. Further, a silicon oxide film is deposited to cover the silicon oxide films, diffusion layers and the gate electrode, as an interlayer insulating film.
Moreover, metal plugs are formed in the silicon oxide film so as to reach the silicide layers. Further, wiring patterns are formed on the upper surface of the metal plugs. Under this condition, the silicide layers are electrically connected to the wiring patterns via the metal plugs. Finally, the entire surface of the device is covered with the silicon nitride film as a passivation film.
When the silicon nitride film is deposited using the plasma CVD method, the silicon nitride film has a relatively small water-permeability in many cases. However, the problem with respect to the hot carrier deterioration can not be solved in the first conventional reference. Namely, active hydrogen radicals take place when the silicon nitride film is formed in a plasma atmosphere containing ammonia and silane. It has been reported that the hydrogen radicals diffuse the gate oxide film to increase the Si--H combinations which becomes the cause of the deterioration.
On the other hand, when an interlayer insulating film, such as the silicon oxide film, is formed by a SOG (spin on glass) film, the interlayer insulating film normally contains slight water. However, the SiN film does not pass the water through because the silicon nitride film has a relatively small water-permeability, as mentioned before. Consequently, the water in the interlayer insulating film mostly diffuses in the direction of the gate oxide film during a final heat treatment in foaming gas.
Thus, when the water diffuses toward the gate oxide film and the spacer oxide film, an electron trap (namely, water related trap) caused by the water in the oxide film. As a result, the hot carrier-resist is largely reduced. This fact has also been described in the above-mentioned 48th symposium lecture with respect to a semiconductor integrated circuit technique (the water diffusion model for the increasing effect of the hot carrier deterioration due to the nitride film passivation, P.134).
To avoid such a problem, another suggestion has been made about a semiconductor device in which the silicon nitride film is placed below the silicon oxide film (namely, the interlayer insulating film). Herein, the above semiconductor device will be referred to as a conventional second reference.
With such a structure, when the silicon nitride film is formed by the use of the thermal decomposition CVD method, the diffusion of the water is suppressed because the generation of the active hydrogen radicals is prevented during the formation of the silicon nitride film.
However, the thermal process temperature which is necessary for the thermal decomposition containing the ammonia and the silane is higher than the deposition temperature of the plasma method. Consequently, the heat resistance of the silicide layer becomes a problem. Namely, the short channel effect must be suppressed by preventing the diffusion of the impurities doped in the diffusion layer to achieve high integration.
When the connection surface of the diffusion layer contacts with the silicide layer, a leak current which is caused by crystal defects is increased, and as a result, the switching operation of the transistor becomes impossible. Therefore, the above silicide layer must be thinned in accordance with the shallow connection of the diffusion layer.
However, when the deposition temperature of the silicon nitride film exceeds the heat resistance of the silicide in case of the thin-film silicide which is necessary to miniaturize the transistor, the silicide layer is aggregated to form a discontinuos film. As a result, disconnection takes place, and the sheet resistance is largely increased.